1. Field of the Invention
The present invention relates to a liquid crystal display device and a liquid crystal projection display device employing a liquid crystal panel for forming optical images as a result of a variation in scattering efficiency.
2. Description of the Prior Art
The research and development of display devices employing liquid crystals has flourished because this technology enables the displays to be more compact and lighter than the conventional display devices using CRT. In recent years liquid crystal display devices have been commercialized using the twisted nematic mode, in which the optical rotary power is modulated by the electric field. These devices are extensively applied to computer monitors, pocket TVs, viewfinders on video cameras, etc.
Liquid crystal projection display devices have also been developed, in which an image on a liquid crystal panel is enlarged and projected with a projection lens onto a large screen. These devices are used for home theaters and other purposes such as presentation.
FIG. 28 shows a perspective view of the major elements of a conventional liquid crystal display device. The light source comprises a fluorescent lamp placed in a fluorescent lamp box 286 and a diffusion plate 287 placed in front of the box. Diffusion plate 287 diffuses the light emitted from fluorescent lamp box 286 to become a plane light source of uniform brightness.
Liquid crystal display device 289 comprises a TN liquid crystal panel that is sandwiched by polarizers 288a and 288b. Polarizer 288a, positioned between diffusion plate 287 and TN liquid crystal panel 289, linearly polarizes the light beams from the plane light source. Polarizer 288b (referred to as an analyzer hereafter), positioned between TN liquid crystal panel 289 and a viewer of the display device, blocks the light beams from TN liquid crystal panel 289 according to the degree of modulation that the light beams experience through TN liquid crystal panel 289. Typically polarizer 288a and analyzer 288b are arranged so that their polarization directions are perpendicular to each other.
To summarize, first, a plane light source is created; the light beams from the plane light source are linearly polarized by polarizer 288a; a TN liquid crystal panel modulates the linearly polarized light beams according to image signals that are applied to the panel; analyzer 288b blocks or passes the light beams according to the degree of modulation, and thus, images are formed on the panel.
Recently, liquid crystal panels are becoming larger with diagonal lengths of over 10 inches, and, accordingly, a suitable back lighting method is needed. It is difficult to illuminate the whole display area uniformly without increasing the thickness of the display device. To solve this problem edge lighting is being used in which, as shown in the cross sectional view of FIG. 29, light is introduced into a light guide 291 from fluorescent lamps 292 placed on both sides. This scheme allows the back light to be as thin as the diameter of the fluorescent lamps. Brightness and uniformity can be improved by adding a diffusion plate, a prism sheet and the like to the light guide.
The conventional viewfinder is described below. An example of a conventional viewfinder is shown in Japanese Patent Laid-Open Publication SHO 62-111233. In the present specification the viewfinder comprises at least an image display device and a light source such as a light emitting diode, both integrated into one.
FIG. 30 is a cross sectional view of a conventional viewfinder. In the figures reference numeral 31 is a body; 32, an ocular cover; 300, an ocular ring; 309, a TN liquid crystal display device. Body 31 encloses a liquid crystal display device and a back light as a light source. Body 31 and ocular ring 300 contain lenses 301 and 302, which, in combination, function as magnifying lenses. Focusing can be changed to suit the viewer's vision by adjusting the depth of insertion of ocular ring 300. TN liquid crystal display device 309 has a liquid crystal layer of approximately 5 .mu.m in thickness and color filters arranged in mosaic structure. Further, polarizer 308a and analyzer 308b sandwich the TN liquid crystal display device. The viewfinder is mounted on the main body of the video camera with fixture 33.
The principle of the operation is the same as that of the TN liquid crystal display device described above. In the case of the viewfinder, however, displayed images are magnified with lenses 301 and 302.
The conventional liquid crystal projection display device is described below. FIG. 31 shows a schematic view of the conventional liquid crystal projection display device. The liquid crystal projection display devices currently available on the market use a TN liquid crystal panel as described above. The TN liquid crystal panel requires a polarizer on each of the entrance and exit sides to modulate the light intensity. Therefore, it inherently has a low light utilization efficiency.
One method to control the light intensity without polarizers uses light scattering. The polymer-dispersed liquid crystal panel in particular is being studied intensively as shown in U.S. Pat. No. 4,435,047 in the expectation of improving its brightness.
FIG. 31 shows a liquid crystal projection display device which has three liquid crystal panels each modulating only one of the RGB lights. There is however, a single-panel liquid crystal projection display device, in which all pixels have one of RGB filters and modulate only the light intensity of the respective color. The single-panel liquid crystal projection display device can be compact and light in weight because it does not need the optics for color separation and mixture that are necessary to the three-panel liquid crystal projection display device. Although the device does not require adjustment of RGB convergence, the poor characteristic of the color filters gives rise to a low color purity for projected images.
U.S. Pat. No. 5,161,042 demonstrates one method for correcting this problem: the white light emitted by a light source is separated into RGB lights, each of which is focused onto individual pixels with microlenses.
A video camera must be light and compact for portability and ease of operability. For this reason liquid crystal display devices are being introduced as viewfinders. The power consumption of the current viewfinders is relatively high because they use a TN liquid crystal display device. For example, there is a viewfinder using a TN liquid crystal display device which consumes 0.1 W for a TN liquid crystal panel and 1.0 W for a light source, that is, 1.1 W in total. The necessity of a video camera to be light and compact imposes a limit on the size of battery it can carry. When a viewfinder uses a large amount of power, its time of continuous operation time becomes correspondingly short.
The same problem exists for a portable laptop computer monitor.
The following factor contributes to the large power consumption of the TN liquid crystal display device. As mentioned above the TN liquid crystal display device needs two polarizers, one placed on the entrance side and the other on the exit side, and the total transmittance is low, approximately 30%. To obtain a necessary brightness, therefore, a large light source is required, which results in a high power consumption.
For a liquid crystal panel, the present invention employs a polymer-dispersed liquid crystal panel that forms optical images as a result of a variation in scattering efficiency. It can provide a bright display device because the absence of polarizers gives rise to a high light utilization efficiency. That is, the light source consumes less power. In a device with a polymer-dispersed liquid crystal panel a higher contrast ratio is obtained when the light beams from a light source have a higher directionality. In a polymer-dispersed liquid crystal panel, a change in the voltage applied to a pixel induces a change in the light scattering of the pixel. When no voltage is applied, the light beams are scattered the most, and as the applied voltage is increased, light scattering decreases. The light beam of a high directionality projected on the liquid crystal panel is modulated in intensity due to light scattering, and a viewer of the panel sees the modulation in light intensity from the pixel. Thus, the viewer sees the change in brightness of the pixel. This is a principle of the display device. It is difficult, however, to illuminate the entire panel uniformly with the light beams of a high directionality. Typically, a long distance is needed between the light source and the liquid crystal panel in order to illuminate the entire panel uniformly. Also, for a larger panel a longer distance is necessary, which result in a larger display device. This eliminates the feature of thinness in the liquid crystal panel. If diffused light is used, on the other hand, the light passing through the polymer-dispersed liquid crystal panel in the transparent state is still diffused light. This situation is much the same as when the polymer-dispersed liquid crystal panel is in the scattering state and it results in a low contrast ratio.
The liquid crystal projection display device employing a polymer-dispersed liquid crystal panel typically uses apertures to pass light beams close to the axis of directionality. This is based on the fact that the light beams exiting from the liquid crystal panel with a limited solid angle vary their intensities more in the light scattering state. That is, a higher contrast ratio is obtained only by improving the light scattering characteristics of the liquid crystal panel or by using a light source of high directionality with proper projection optics having a small collection angle.
In particular, a single-panel liquid crystal projection display device employing a polymer-dispersed liquid crystal panel as a light valve, as shown in U.S. Pat. No. 5,161,042, needs a projection lens having a large collection angle to collect the RGB light beams passing through each pixel.